Technical Field
The present disclosure relates to an apparatus for processing a wireless signal, for use in a wireless communication system, and more particularly, to a radio frequency filter with a cavity structure, such as a cavity filter.
Description of the Related Art
A radio frequency filter with a cavity structure generally includes a plurality of rectangular accommodating spaces, that is, cavities in a metal housing, with a resonant element such as a dielectric resonant (DR) element or a metal resonant rod accommodated in each cavity, to thereby generate ultra-high frequency resonance. In the radio frequency filter with this cavity structure, a cover may be provided on the cavity structure to cover the cavities, and a tuning structure with a plurality of tuning screws and nuts for fastening the screws may be installed on the cover in order to tune filtering characteristics of the radio frequency filter. An exemplary radio frequency filter with a cavity structure is disclosed in Korea Laid-Open Patent Publication No. 10-2004-100084 (entitled ‘Radio Frequency Filter’, publicized on Dec. 2, 2004, and invented by PARK Jong Gyu, et. al.) filed by the present applicant.
The radio frequency filter with a cavity structure is used to process a transmission/received wireless signal in a wireless communication system, particularly in a base station or a relay in a mobile communication system.
Meanwhile, Korea Laid-Open Patent Publication No. 10-2014-0026235 (entitled ‘Radio Frequency Filter with Cavity Structure’, publicized on Mar. 5, 2014, and invented by PARK Nam Sin, et. al.) filed by the present applicant discloses a simplified filter structure for enabling frequency tuning without using a coupling structure of tuning screws and fastening nuts. The document proposes a technology of forming one or more sunken portions at positions corresponding to resonant elements on a cover in the process of fabricating the cover using a plate of a base material such as aluminum or magnesium (including an alloy) by pressing or die casting. Also, a plurality of dot peens are formed in the sunken portions by marking or pressing the cover using a marking pin of an external marking equipment. These sunken portions and dot peens substitute for the coupling structure of tuning screws and fastening nuts, which is generally used for frequency tuning, and enable appropriate tuning by reducing the distance between the sunken portions (and the dot peens) and the resonant elements.
The technology disclosed in Korea Laid-Open Patent Publication No. 10-2014-0026235 is suitable for a small, lightweight filter structure because it does not adopt the general coupling structure of tuning screws and fastening nuts. According to the technology disclosed in Korea Laid-Open Patent Publication No. 10-2014-0026235, however, the sunken portions should be formed on the cover by die casting, when a relatively large filter is fabricated. As a result, process cost may be increased.
Moreover, the cover and a housing are fabricated of a lightweight material such as aluminum (including an alloy) in consideration of strength, weight, fabrication cost, and task easiness in the technology disclosed in Korea Laid-Open Patent Publication No. 10-2014-0026235. Due to a large thermal expansion coefficient of aluminum, a change in ambient temperature and heat emission of the product cause a change in the characteristics of the filter.
More specifically, an antenna device with a filter is generally used in a use environment of constant temperature and high temperature and affected by heat emitted from other parts (for example, an amplifier). Especially if a cavity filter is used as a high-power transmission filter, a large amount of heat is produced in view of insertion loss. If ambient temperature is changed, the housing and resonator of the cavity filter causes thermal contraction and expansion. As capacitance and inductance are changed due to a change in the distances between components and thus unique characteristics of the filter are changed, operation malfunction may occur. This problem becomes serious in a resonator structure using a metal resonant rod.
In this context, various techniques have been studied and adopted in order to minimize temperature change-incurred characteristic changes in the resonator structure of a conventional cavity filter, particularly a structure using a metal resonant rod. For example, the resonant rod is basically formed of a material having a very small thermal expansion coefficient such as Invar, or each resonant element has a lower part formed of the same material as the housing (for example, aluminum) and an upper part formed of a different material from that of the lower part, such as Bs, Sum, Cu, or the like. However, it is difficult to compensate the temperature of the radio frequency filter because of the limitations (price and thermal expansion coefficient) of a material applied to the resonant rods of the cavity filter.
The contents described as the related art have been provided merely for assisting in the understanding for the background of the present invention and should not be considered as corresponding to the related art known to those skilled in the art.